Solar cells are used to convert the energy of sunlight into electrical energy using the photoelectric effect. Depending on the absorber material, however, only a certain part of the solar spectrum may be used efficiently. For example, if the absorber is made of a semiconductor material, photons with energies below the band gap may not contribute directly to the generation of electron-hole pairs and are therefore hardly absorbed. Photons with energies far above the band gap generate electron-hole pairs, but electrons and holes from the excited higher-energy states thermalize into low-energy states near the band edges and therein release the energy difference in the form of heat.
One way to minimize these two loss mechanisms in the area of high- and low-energy photons is to use two or more absorbers made of different materials that efficiently convert radiation energy from different areas of the solar spectrum into electrical energy. The absorber or absorbers that efficiently use high-energy photons for the photovoltaic energy conversion and efficiently transmit the photons with lower energies to the underlying absorber(s) are located on a side of the solar cell facing the solar irradiation.
This solar cell structure, known as a tandem solar cell or multijunction cell, has long been known, for example from the U.S. Pat. No. 4,496,788, and has already been commercialized in the area of highly efficient space solar cells or in the area of terrestrial solar cells, where sunlight is concentrated via optics onto a small area.
It would also be desirable to be able to also use tandem solar cells for terrestrial, non-concentrating photovoltaic applications. To achieve this, however, the high manufacturing and material costs of conventional tandem solar cells must be significantly reduced. For this purpose, tandem solar cells may be based on relatively inexpensive and relatively efficient (mono- or multi-) crystalline silicon solar cells, i.e. one or more absorbers made of materials other than crystalline silicon may be applied to the silicon absorber. Materials would be possible which allow photovoltaic energy conversion for photon energies below the silicon band gap of 1.12 eV, wherein such an absorber would be applied to the side of the silicon absorber facing away from solar radiation. Alternatively, absorbers made of materials that efficiently transmit photons with energies below 1.4 eV and efficiently use higher-energy photons for the photovoltaic energy conversion could be applied to the side of the silicon absorber facing the solar irradiation.
Silicon solar cells with only one pn-junction are constantly being developed further. A significant improvement in energy conversion efficiency may be achieved by improving the quality of the surface passivation. Dielectric passivation layers, such as silicon oxides (SiOx), silicon nitrides (SiNx) or aluminum oxides (Al2O3), electrically insulate the respective covered areas, so that additional non-passivated contacts are required for the extraction of charge carriers. Typically, a metal borders locally directly on the crystalline silicon, forming a defective and therefore highly recombination-active interface. In contrast, so-called charge carrier selective contacts offer the possibility not only of passivating the surfaces covered with them excellently, but also of selectively extracting one charge carrier type, i.e. either electrons or holes, from the silicon absorber in these areas.
The term “charge carrier selective contact” comprises different embodiments. The best known for this in the field of silicon photovoltaics are heterojunctions between amorphous, hydrogen-rich silicon and the crystalline silicon absorber. However, the electrical properties of these heterojunctions often degrade strongly at temperatures above 200° C. An alternative known from bipolar electronics, which has been increasingly investigated in photovoltaics in recent years, are charge carrier selective contacts consisting of a thin interfacial oxide (or more generally from an interfacial dielectric) and a layer of doped amorphous or partially crystalline or polycrystalline silicon deposited thereon. The interfacial oxide is partly also called tunnel oxide, wherein the dominant physical current transport mechanism through the oxide is still the subject of current scientific debates. Depending on whether the amorphous or partially crystalline or polycrystalline silicon layer on the interface oxide is doped n or p, the charge carrier selective contact is either transparent, respective permeable, for electrons or holes and blocks the other charge carrier type near the interface between the oxide and the silicon absorber. Charge carrier selective contacts consisting of a thin interfacial oxide and a layer of doped amorphous or partially crystalline or polycrystalline silicon deposited thereon are usually stable at process temperatures up to at least 1050° C. Their excellent passivation quality is typically independent of the type of layers on them.